Exploring Multi-Anion Chemistry in Yttrium Oxyhydrides: Solid-State NMR Studies and DFT Calculations

[Image: see text] Rare earth oxyhydrides REO(x)H((3–2x)), with RE = Y, Sc, or Gd and a cationic FCC lattice, are reversibly photochromic in nature. It is known that structural details and anion (O(2–):H(–)) composition dictate the efficiency of the photochromic behavior. The mechanism behind the photochromism is, however, not yet understood. In this study, we use (1)H, (2)H, (17)O, and (89)Y solid-state NMR spectroscopy and density functional theory (DFT) calculations to study the various yttrium, hydrogen, and oxygen local environments, anion oxidation states, and hydride ion dynamics. DFT models of YO(x)H((3–2x)) with both anion-ordered and anion-disordered sublattices are constructed for a range of compositions and show a good correlation with the experimental NMR parameters. Two-dimensional (17)O–(1)H and (89)Y–(1)H NMR correlation experiments reveal heterogeneities in the samples, which appear to consist of hydride-rich (x ≈ 0.25) and hydride-poor domains (x ≈ 1) rather than a single composition with homogeneous anion mixing. The compositional variation (as indicated by the different x values in YO(x)H((3–2x))) is determined by comparing static (1)H NMR line widths with calculated (1)H–(1)H dipolar couplings of yttrium oxyhydride models. The 1D (17)O MAS spectrum demonstrates the presence of a small percentage of hydroxide (OH(–)) ions. DFT modeling indicates a reaction between the protons of hydroxides and hydrides to form molecular hydrogen (H(+) + H(–) → H(2)). (1)H MAS NMR indicates the presence of a mobile component that, based on this finding, is attributed to trapped molecular H(2) in the lattice.